The field of free-form fabrication has seen many important recent advances in the fabrication of articles directly from computer-aided design (CAD) data bases. These advances, many of which are in the field of rapid prototyping of articles such as prototype parts and mold dies, have greatly reduced the time and expense required to fabricate articles, particularly in contrast to conventional machining processes in which a block of material, such as a metal, is machined according to engineering drawings. The time required to produce prototype parts from engineering designs has reduced from several weeks, using conventional machinery, to a matter of a few hours. In addition, the complexity and accuracy of the articles which may now be fabricated according to these new technologies is much improved over that available from conventional machining.
One example of a modern rapid prototyping technology is the selective laser sintering process practiced by systems available from DTM Corporation of Austin, Tex. According to this technology, articles are produced in layerwise fashion from a laser-fusible powder that is dispensed one layer at a time. The powder is fused, or sintered, by the application of laser energy that is directed in raster scan fashion to portions of the powder layer corresponding to a cross-section of the article. After the fusing of powder in each layer, an additional layer of powder is then dispensed, and the process repeated, with fused portions of later layers fusing to fused portions of previous layers (as appropriate for the article), until the article is complete. Detailed description of the selective laser sintering technology may be found in U.S. Pat. No. 4,863,538, U.S. Pat. No. 5,017,753, U.S. Pat. No. 5,076,869, and U.S. Pat. No. 4,944,817, all assigned to Board of Regents, The University of Texas System and incorporated herein by this reference, and in U.S. Pat. No. 4,247,508 and U.S. Pat. No. 5,352,405, both assigned to DTM Corporation and incorporated herein by this reference. The selective laser sintering technology has enabled the direct manufacture of three-dimensional articles of high resolution and dimensional accuracy from a variety of materials including wax, polycarbonate, nylon, other plastics, and composite materials such as polymer coated metals and ceramics. Wax parts may be used in the generation of tooling by way of the well-known "lost wax" process. Examples of composite powder materials are described in U.S. Pat. No. 4,944,817, U.S. Pat. No. 5,156,697, and in U.S. Pat. No. 5,284,695, all assigned to Board of Regents, The University of Texas System and incorporated herein by this reference.
The field of rapid prototyping of parts has, in recent years, made large improvements in providing high strength, high density, parts for use in the design and pilot production of many useful articles, including metal parts. These advances have permitted the selective laser sintering process to now also be used in fabricating prototype tooling for injection molding, with expected tool life in excess of ten thousand mold cycles.
The production of metal parts by selective laser sintering conventionally uses a powder of metal particles coated with a polymer, from which a "green" part is fabricated by selective laser sintering of the polymer coating to binds the particles to one another. The green part is then heated to a temperature above the decomposition temperature of the polymer, which both drives off the polymer and also binds the metal substrate particles to one another to form the metal article. In the case where the article is a mold die for injection molding, the die is also impregnated with another metal, for example copper, to add strength, wear resistance, and tooling life. According to another conventional approach, composite polymer-metal parts or tooling may be formed without subjecting the green part to a post-process anneal. In this technique, the green parts formed by the selective laser sintering of a polymer-coated metal powder are impregnated with a liquid resin. The resin is cross-linked, either at room temperature or at an elevated temperature, depending upon the resin chemistry, resulting in near-fully dense composite articles.
The selective laser sintering technology has also been applied to the direct fabrication of articles, such as molds, from metal powders without a binder. Examples of metal powder reportedly used in such direct fabrication include two-phase metal powders of the copper-tin, copper-solder (the solder being 70% lead and 30% tin), and bronze-nickel systems.
The metal articles formed by selective laser sintering in these ways have been quite dense, for example having densities of up to 70-80% of fully dense (prior to any infiltration). However, some porosity still remains in these metal articles, thus limiting their structural strength and thus the utility of the articles. For example, the less-than-fully dense articles so fabricated, even when subsequently infiltrated by a molten metal, will not have the same length of life, when used as injection mold dies, as will machined dies from solid metal substrates. Such articles are also not useful as actual working parts in machinery, in comparison to machined parts.
It has been observed that the direct selective laser sintering of metal to produce a metal, fully-dense, article encounters many significant technical barriers. Firstly, since the melting, or sintering, temperature of metals is typically very high, the laser power required to fuse the metal into a fully-dense article is also very high. Secondly, the laser fusing of a large area of a metal powder layer into a mass necessarily presents very large thermal gradients in the selective laser sintering system, greatly improving the likelihood of distortion of the part due to thermal effects such as curling and growth. While these gradients may be reduced by raising of the ambient temperature of the selective laser sintering chamber to near the sintering temperature of the metal powder, this unfortunately causes caking of the powder as it is dispensed into the chamber, further limiting the ability of this technology to directly produce fully-dense metal articles.
By way of further background, another laser-based rapid prototyping method is described in Klocke, et al., "Rapid Metal Prototyping and Tooling", EARP-Newsletter, Vol. 6, (July 1995) and is referred to as "laser generating". According to this technique, the powder is not dispensed in a layer-wise fashion as in the case of selective laser sintering, but is instead fed into the melting pool produced by the location at which the laser is focused. The fed powder is then bonded to the article at that point, with additional powder fed thereto to further build up the article. The Klocke et al. article indicates that thin-walled articles have been formed from metal powders (iron and cobalt base metals) using the laser generating technology.
By way of further background, the use of hot isostatic pressing (HIP) to form metal articles is well known. According to this technique, a porous metal article is surrounded by a machined gas-impervious shell, or "can", also preferably formed of a metal. Alternatively, a can may be filled with a metal powder that is not previously formed into a shape. The article in the can is placed into a pressure vessel within a furnace. The HIP process elevates the temperature of the workpiece to a sufficient temperature so that the metal reaches a softened state, while pressurizing the workpiece with an inert gas. The pressure exuded by the gas is isostatic, and exerts an equal force in all directions against the gas-impervious shell. The softened metal of the workpiece (and can) is thus isotropically squeezed, eliminating any porosity in the article. A fully-dense metal article, with some minimal amount of shrinkage from its pre-HIP state, thus results from the HIP processing. The can is then conventionally removed from the article, for example by machining or by etching. However, as is well-known in the HIP field, the fabrication of the "can" is generally quite difficult, when accomplished by conventional techniques such as machining. As such, the dimensional resolution and shape-complexity of the articles produced by HIP processes are typically limited.
It is therefore an object of the present invention to provide a method of fabricating fully-dense metal articles in a way which takes advantages of the resolution and object complexity enabled by selective laser sintering.
It is a further object of the present invention to provide such a method in which HIP-processed articles may be fabricated, as fully-dense articles, but with improved accuracy and increased complexity relative to conventional HIP articles.
It is a further object of the present invention to provide such a method in which the articles may be formed and subjected to HIP processing, but where the "can" is formed of the same material as the underlying article, and may be left on the article after fabrication is completed.
It is a further object of the present invention to provide such a method in which fully-dense metal articles may be produced with the complexity and accuracy of laser-directed processes, from a wide range of metal materials.
Other objects and advantages of the present invention will be apparent to those of ordinary skill in the art having reference to the following specification together with its drawings.